Shock-Tube Studies on the Reactions of CF₂(XA₁) with O(³P) and H Atoms

Abstract

The reactions of CF2(X1A1) radicals with O(3P) and H atoms have been studied by using a shock tube/atomic resonance absorption spectroscopy technique over the temperature ranges 2000−2430 and 1450−1860 K and the total density range 6.1 × 1018 to 1.2 × 1019 molecules cm-3. Nitrous oxide and ethyl iodide were used as precursors of O(3P) and H atoms, respectively. Electronically ground state CF2(X1A1) radicals were produced through the thermal decomposition of chlorodifluoromethane. The rate coefficients for the reactions CF2(X1A1) + O(3P) and CF2(X1A1) + H were obtained from the decay profiles of O and H atom concentrations as k(CF2+O) = 10-10.39±0.07 and k(CF2+H) = 10-10.18±0.21 exp[−(19.0 ± 6.7) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the two standard deviation level). Neither rate coefficient had any pressure dependence under the present experimental conditions. The G2-level ab initio molecular orbital calculation was also performed to examine the product channels for the CF2(X1A1) + O(3P) and CF2(X1A1) + H reactions. The theoretical calculation showed that the most energetically favorable pathways for CF2(X1A1) + O(3P) and CF2(X1A1) + H systems were the channels producing FCO + F and CF + HF, respectively. The G2 energy of the transition state for the channel CF2(X1A1) + O(3P) → FCO + F was 116 kJ mol-1 lower than that of the reactants CF2(X1A1) + O(3P), while the energy of the three-centered transition state for the channel CF2(X1A1) + H → CF + HF is 45 kJ mol-1 higher than that of the reactants CF2(X1A1) + H. These results could qualitatively explain the difference of the temperature dependence observed between k(CF2+O) and k(CF2+H).